package algorithm.problems.dynamic_programming;

/**
 * Created by gouthamvidyapradhan on 01/04/2017.
 * A message containing letters from A-Z is being encoded to numbers using the following mapping:
 * <p>
 * 'A' -> 1
 * 'B' -> 2
 * ...
 * 'Z' -> 26
 * Given an encoded message containing digits, determine the total number of ways to decode it.
 * <p>
 * For example,
 * Given encoded message "12", it could be decoded as "AB" (1 2) or "L" (12).
 * <p>
 * The number of ways decoding "12" is 2.
 */
public class DecodeWays {
    /**
     * Main method
     *
     * @param args
     * @throws Exception
     */
    public static void main(String[] args) throws Exception {
        System.out.println(new DecodeWays().numDecodings("3120"));
    }

    public int numDecodings(String s) {
        if (s == null || s.isEmpty()) return 0;
        int[] dp = new int[s.length() + 2];
        dp[s.length()] = 1;
        dp[s.length() + 1] = 1;
        for (int i = s.length() - 1; i >= 0; i--) {
            for (int j = i + 1; j < i + 3; j++) {
                if (j <= s.length()) {
                    String subStr = s.substring(i, j);
                    if (!subStr.startsWith("0")) {
                        int intVal = Integer.parseInt(subStr);
                        if (intVal <= 26) {
                            dp[i] += dp[j];
                        }
                    }
                }
            }
        }
        return dp[0];
    }
}
